Ultra-Reliable and Low-Latency Communications (URLLC) is a category of services defined in 3GPP TR 22.862, Version 14.1.0. For URLLC services, both high reliability and low latency are required. However, these requirements are mutually conflicting and are typically traded off against each other, which brings a remarkable challenge to user-plane (UP) design.
According to 3GPP TR 22.862, the latency requirement for URLLC services ranges from 1 ms to 10 ms for various applications including automation applications, smart grids and intelligent transportation. The reliability requirement for URLLC services ranges from a residual error rate of 10−4 to 10−6, or even to 10−9. It is to be noted here that in calculating the residual error rate, packets arriving later than the required latency bound, such as 1 ms or 10 ms, will be regarded as errors in the context of URLLC.
Simultaneously achieving such high requirements on both reliability and latency may affect several layers and components in both Radio Access Network (RAN) and Core Network (CN). The URLLC can be considered as an extremely high Quality of Service (QoS) use case for both RAN and CN.
In order to meet the above requirements, it has been proposed to provide a terminal device (e.g., a User Equipment, or UE) with multiple connections to multiple network devices (e.g., evolved NodeBs (eNBs)). This is particularly useful when the terminal device is communicating time-critical data and/or is in poor network coverage (e.g., at cell edge), since the diversity gain provided by the multiple connections can be fully exploited.
FIG. 1 shows an exemplary scenario where such multi-connection transmission is deployed. As shown, a terminal device 110 has uplink (UL) connections with three network devices 120, 122 and 124. In particular, the terminal device 110 transmits UL data to the network devices 120, 122 and 124 over bundled Transmission Time Intervals (TTIs). Successive TTIs are bundled for improving transmission reliability, with each TTI for transmitting the same information to achieve a transmission diversity gain. In the example shown in FIG. 1, six successive TTIs are bundled. The first three of the TTIs, labeled as 1-1, 1-2 and 1-3, are allocated for transmission towards the network device 120, the following two TTIs, labeled as 2-1 and 2-2, for transmission towards the network device 122, and the last TTI, labeled as 3, for transmission towards the network device 124. Optionally, a flexible beamforming scheme can be adopted at the terminal device 110, such that the transmissions towards the network devices 120, 122 and 124 can be carried out via different beams (B1, B2 and B3 as shown in FIG. 1), respectively.
In order to achieve UL synchronization at a network device, a Timing Advance (TA) value is configured for a terminal device for UL transmission towards the network device. However, in the multi-connection transmission scenario, different TA values for different network devices at one single terminal device may become problematic.
FIG. 2 shows an exemplary TA configuration for a terminal device (e.g., the terminal device 110) in a multi-connection transmission scenario (e.g., the scenario shown in FIG. 1). The uppermost line of FIG. 2 shows UL timing at each of the network devices 120, 122 and 124 (it is assumed here that the network devices 120, 122 and 124 have their uplinks synchronized with each other). The three lines below show respective UL transmission timings for the network devices 120, 122 and 124 at the terminal device 110. The TA values for the network devices 120, 122 and 124 are denoted as TA1, TA2 and TA3, respectively and it is assumed here that TA2>TA1>TA3. It can be seen from FIG. 2 that a portion of the last TTI 1-3 for transmission to the network device 120 overlaps a portion of the first TTI 2-1 for transmission to the network device 122 in time, as shown in the hatched regions. That is, since TA2 is larger than TA1, the transmission to the network device 122 is scheduled to begin before the transmission to the network device 120 ends. This is not possible for the terminal device 110 if it has only one radio unit, especially when the beamforming is applied.
There is thus a need for an improved solution for multi-connection transmission with different TA values.